The purpose of Dynamic Range Compression (DRC) is to reduce the dynamic range of an audio signal. A time-varying gain factor is applied to the audio signal. Typically, this gain factor is dependent on the amplitude envelope of the signal used for controlling the gain. The mapping is in general non-linear. Large amplitudes are mapped to smaller ones while faint sounds are often amplified. Scenarios are noisy environments, late night listening, small speakers or mobile headphone listening.
A common concept for streaming or broadcasting Audio is to generate the DRC gains before transmission and apply these gains after receiving and decoding. The principle of using DRC, i.e. how DRC is usually applied to an audio signal, is shown in FIG. 1A. The signal level, usually the signal envelope, is detected, and a related time-varying gain gDRC is computed. The gain is used to change the amplitude of the audio signal. FIG. 1B shows the principle of using DRC for encoding/decoding, wherein gain factors are transmitted together with the coded audio signal. On the decoder side, the gains are applied to the decoded audio signal in order to reduce its dynamic range.
For 3D audio, different gains can be applied to loudspeaker channels that represent different spatial positions. These positions then need to be known at the sending side in order to be able to generate a matching set of gains. This is usually only possible for idealized conditions, while in realistic cases the number of speakers and their placement vary in many ways. This is more influenced from practical considerations than from specifications. Higher Order Ambisonics (HOA) is an audio format allows for flexible rendering. A HOA signal is composed of coefficient channels that do not directly represent sound levels. Therefore, DRC cannot be simply applied to HOA based signals.